1. Field of the invention
This invention relates to a motor for use with a video tape recorder and the like. More particularly, the invention relates to the construction of a pulley on such motor.
2. Related art
A brushless capstan motor is an example of the motors that are conventionally used to drive tape on a video tape recorder and the like. A prior art example of this brushless capstan motor is shown in FIG. 25. As shown, a rotor case 121 in the form of a shallow cup is furnished with a cylindrical boss portion 124 in the center. A rotating shaft 123 is pressed into a cylindrical hole 122 through the boss portion 124 and the shaft is integral with the rotor case 121. A rotor magnet 115 is fitted on the inner surface of the peripheral wall of the rotor case 121 in such a way that it faces the salient poles of a stator core 116 with a certain clearance being provided. Therefore, if coils 117 wound around the salient poles of the stator core 116 are supplied with an electric current in a controlled manner, the rotor magnet 115 is energized and both the rotor case 121 and the rotating shaft 123 are driven to rotate. A pulley 125 is fixed in engagement with the outer circumference of the boss portion 124. A peripheral groove 126 having a rectangular cross section is formed around the entire circumference of the pulley 125 and a belt (not shown) is stretched in the peripheral groove 126. As the rotor case 121 and the rotating shaft 123 rotate, the pulley 125 also rotates, transmitting the rotating force to the drive mechanism for the reel mount and the like via the belt.
Another example of the brushless capstan motor is shown in FIG. 26. As shown, a rotor case 129 in the form of a shallow cup is slightly raised in the central portion to provide an elevation 129a, which is also raised up in the center in a semielliptical form to provide an elevation 129b. A through-hole is formed in the center of the elevation 129b and a rotating shaft 127 is pressed into this through-hole. The outer circumference of the elevation 129a is in engagement with the inner circumference of the through-hole in the center of the pulley 128. As in the first example, a peripheral groove 130 is formed around the circumference of the pulley 128. A plurality of projections 128a are formed on the underside of the pulley 128 and inserted into through-holes formed in the top surface of the rotor case 129. The distal ends of the respective projections 128a that protrude beyond the through-holes are welded so that the pulley 128 is secured to the rotor case 129 to form an integral assembly. A belt (not shown) is stretched in a peripheral groove 130 in the pulley 128 and driven by causing the pulley to rotate as a member integral with the rotor case 129.
Speaking of the motor shown in FIG. 25, the part which secures the pulley 125 is composed of three members, the rotor case 121, the boss portion 124 and the rotating shaft 123. Since the errors in precision of these parts will add to each other, the pulley 125 may experience torsion relative to the rotating shaft 123, making it difficult to provide a motor having satisfactory rotating performance unless those parts are worked to significantly improved precision. If the pulley 125 experiences torsion, the belt will either vibrate or be driven at uneven speeds. A further problem with the torsion of the pulley is that the load on the belt tension fluctuates, causing the sliding characteristics of a sintered bearing 120 and the rotating shaft 123 to deteriorate with time. If such a defective motor is used on a video tape recorder and the like, various troubles will occur as exemplified by unevenness in the tape forwarding and reversing operations and the production of wows and flutters during video recording and reproducing operations.
To achieve satisfactory rotating performance with the motor shown in FIG. 26, the rotor case 129 must be worked to high precision. However, the rotor case 129 which is made by pressing is not easy to work with higher precision and if one attempts to increase the working precision, the production cost will increase. A further problem is caused by the fact that the areas of the pulley 128 where the projections 128a are welded to the rotor case 129 to fix the pulley 128 are located in a lower position than the area where the belt is stretched. On account of the tension as applied by the driven belt, the pulley 128 is constantly subjected to a force that works in the direction where the projections 128a will be disengaged from the rotor case 129 on the side where the belt is stretched, with the fulcrum lying at the projections 128a on the side where the belt is not stretched. Hence, the pulley 128 tends to experience deformation or torsion and troubles such as wows and flutters will occur. In the worst case, the pulley 128 will break, making is impossible for the video tape recorder and the like to work normally.